Objectives 426 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

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1 Objectives Explain what is meant by the curvature and focal length of mirrors and lenses. Explain how curvature and focal length are related. Use light rays to trace light from an object to a mirror to the image formed by the mirror. Define index of refraction. Show how Snell s law is used to model refraction at an optical interface. Use light rays in a drawing to show how light is refracted at an optical boundary between air and another medium. Explain what s meant by the focal length of a lens. Use light rays to trace light from an object through a lens to an image formed by the lens. To find out more about reflection and refraction, follow the links at Light, whether thought of as photons or as waves, travels in straight lines through space or a uniform medium. Objects like mirrors can change the path of a light ray to a new straight line. Light can change direction to a new straight line by reflecting off a polished surface. 426 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

2 Figure 10.7 Shadows seem to indicate that light travels in straight lines or rays. If light encounters an interface that changes the medium, such as from air to glass, it may change direction to a new straight line. These processes that make light change direction are called reflection and refraction. We represent the path of a photon through an optical system by a series of straight arrows called rays. The study of light motion in optical systems by using rays is called ray optics. Reflection When a light ray strikes the surface of a material, some of the light may bounce off the surface, much like a billiard ball bounces off the edge of a billiard table. This process is called reflection. Reflected light acts a lot like a billiard ball. Both the ball and the light ray hit and bounce off the surface at equal angles. In Figure 10.8, the light ray is shown to reflect from the surface of the table at the same angle at which it strikes the table. Figure 10.8 Light strikes and reflects from a surface at the same angle. The angles of the incident and reflected light rays are measured from the normal line at the point of reflection. The normal line is a line perpendicular to the surface at the point of reflection. The angle of incidence is the angle between the incoming light ray and the normal line. The angle of reflection is the angle between the normal line and the reflected ray. SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 427

3 From observations and experiments, we know that the angle of incidence is always equal to the angle of reflection. That is, light reflects at the same angle at which it strikes the smooth surface of a material. This is called the law of reflection. Note that the incident ray, the normal, and the reflected ray all lie in the same plane. This plane is called the plane of incidence. The amount of light that s reflected from a surface varies a lot. You ve probably noticed that the polished surface of a new car reflects much more light it appears brighter than the surface of a rusted, dirty surface. Highly polished surfaces used to reflect light are called mirrors. Mirror surfaces of aluminum and silver reflect about 90% of the light that strikes them. Figure 10.9 First- and second-surface mirrors Most common mirrors are made by putting a thin metal coating on glass. The glass provides the flat surface, but the metal makes the surface highly reflective. Smaller amounts of light can be reflected from an uncoated glass surface. If the metal coating is on the first surface the light hits, the mirror is a first-surface mirror. Most very-high-quality mirrors in optical systems are first-surface mirrors. Common household mirrors have the glass in front of the coating to provide protection for the thin metal coating. A common household mirror is a second-surface mirror. Mirrors are further classified by the shape of the surface. Mirrors with flat surfaces are called plane mirrors. Mirrors with spherical reflecting surfaces are called spherical mirrors. As shown in Figure 10.10, an object is a source of light rays. It may emit light, as in a lamp or a laser, or it may be illuminated by an unknown source, as in Figure When you view the reflection of an object in a mirror, you see an image. A plane mirror produces an image that is reversed (right becomes left and vice versa). The image formed by a plane mirror appears to be located the same distance behind the mirror as the object is in front of the mirror (s o = s i ). 428 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

4 Figure A plane mirror reverses left and right. Light leaves the object, reflects off the mirror, and forms an image. When parallel light rays strike the flat surface of a plane mirror, the reflected rays leave the mirror as a parallel beam. Notice in Figure that each of the rays a, b, c obeys the law of reflection. Figure Parallel light rays striking and reflecting from the flat surface of a plane mirror Some mirrors have spherical reflecting surfaces. Spherical mirrors can be convex, like the outside surface (or back) of a spoon, or concave, like the inside surface of a spoon. You ve probably seen convex mirrors in stores, where they are used for security. Many cars and trucks have convex spherical rearview mirrors on the passenger sides. These mirrors provide wide-angle views for the drivers, at the price of making things look closer than they are. When parallel light rays strike the surface of a concave mirror, they re reflected to a point in front of the mirror. (See Figure on the next page.) Why does this happen? The parallel rays are all reflected according to the law of reflection. But notice that the normal line always a radius of the mirror changes its direction at each point along the curved surface of the mirror. The changing normal line causes the reflected rays to bounce off as nonparallel lines. Thus, the incident rays are parallel but the reflected rays are not. The reflected rays come to a focus in front of the mirror and continue outward. The point in front of the mirror at which the reflected rays all meet is called the focal point. (Actually, a mirror or lens with a spherical SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 429

5 shape does not have an exact focal point. All the reflected rays converge to a small volume around the focal point. For a mirror or lens that has a radius of curvature that is large compared to the size of the mirror or lens, the volume is small enough to be considered a point. When a very precise focal point is required, a parabola-shaped surface is used instead of a spherical surface.) Figure The focal point of a concave mirror is in front of the mirror. The focal length is ½ the radius of curvature. The focal length f of a spherical mirror is ½ the radius of curvature r. f = 1 2 r Example 10.1 Calculating the Focal Length of a Mirror Find the focal length of a concave mirror with a radius of curvature of r = 16 cm. Solution: f = ½r f = ½(16 cm) f = 8 cm When parallel light rays strike the surface of a convex mirror, they re reflected back along spreading or divergent paths. They re all reflected according to the law of reflection. Again, the orientation of the normal line changes at each point along the curved surface of the mirror. Just as for concave mirrors, this changing normal line causes the reflected rays to be nonparallel. See Figure After reflection, they diverge, as shown. To someone looking at the surface of the mirror, the light rays seem to come 430 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

6 from a point F located behind the mirror. The point is the focal point F for the mirror and is behind the convex mirror. Figure The focal point of a convex mirror is behind the mirror. The focal length is negative. We see most objects by reflected light. For example, when light from the sun strikes a tree, the light is reflected off the surface of the tree. Some of the reflected light enters our eye. Our eye forms an image of the tree that is interpreted by our brain as sight. Figure Reflected light creates most of the images we see. Figure shows light from an object striking a mirror and reflecting through the focal point of the mirror. The reflected rays form an image of the object. This is called a real image, because light rays converge to pass through the image. You can see a real image if you place a small screen or piece of paper at the point at which the rays converge. Figure shows light from an object striking a mirror and reflecting away from the focal point. The reflected rays appear to diverge from the focal point, which is located behind the mirror. There are no light rays actually behind the mirror, so the image that appears to cause the reflection is called a virtual image. You cannot see a virtual image on a screen or piece of paper since no light rays actually converge at the image. SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 431

7 In Figure 10.15, two light rays are traced from an object to its image. An incident ray (ray 1), parallel to the optical axis, is reflected to pass through the focal point F. An incident ray (ray 2) passing through the focal point is reflected parallel to the optical axis. The two rays start from a point at the top of an object point P. The point P where the two rays meet is the location of the real image of point P. All other points on the tree have similar image points that make up a complete image of the tree. You can see the real image if you place a small screen or piece of paper at the location of the image. Figure Ray tracing from an object to a concave mirror and on to an image point Figure also shows the distance from the object to the vertical plane passing throught the vertex V of the mirror. This is called the object distance s o. The image distance, s i, is also shown. The object and image distances are related to the focal length f by the mirror equation: = s s f o When you use this equation, be sure to use the following signs (+ or ) for s o, s i, and f : 1. The object distance s o and image distance s i are positive for real objects and images. They are negative for virtual objects and images. 2. The focal length f and radius of curvature r are positive if the center of curvature lies on the side of the mirror from which the light comes. They are negative if the center of curvature lies on the side where there is no light. For example, in Figure 10.15, s o, s i, and f are all positive. In Figure 10.13, s o is positive but s i and f are negative. i 432 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

8 Example 10.2 Image Distance for a Concave Mirror A concave mirror such as in Figure has a focal length of 25 cm. An object is located 35 cm in front of the mirror on the optical axis. Where will the image of the object be found? Solution: The light comes from the left, and the center of curvature is on the left. Therefore, f is positive. The object is real, so s o is positive. Use the mirror equation. Thus, = s s f o = 35 cm s 25 cm i i = = = s 25 cm 35 cm i s = 87.5 cm or about 88 cm i The image is on the left side of the mirror, 88 cm away from the mirror vertex. Suppose the mirror in Example 10.2 is convex. Can you show that s i = 14.6 cm? Is the image real or virtual? Ray tracing for a convex mirror is similar, as shown in Figure Ray 1 is parallel to the optical axis and reflects so that its extension passes through the focal point. Ray 2 is directed to the focal point, but reflects parallel to the optical axis. The two reflected rays diverge, as if they came from a point behind the mirror. The image is virtual. Figure Ray tracing for a convex mirror SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 433

9 Refraction When light strikes a mirrored surface, it is reflected. When light strikes a different kind of surface like transparent glass, plastic, or water some light is reflected and some light passes through the material. But when light passes from one material to another the direction of travel of the light changes. The amount of the change depends on the angle at which the light hits the interface and on the speed at which the light travels through each material. This change of direction at an interface between two materials is called refraction. Refraction is the process that enables us to make lenses for eyeglasses, telescopes, cameras, and microscopes. Figure Refraction of light makes a straw appear to be bent. You have seen refracted light many times. One common example is the way a drinking straw that s placed in a glass of water looks bent at the surface of the water. This appearance is the result of a change in the direction of light as it goes from water to glass to air. Recall that the speed of light in vacuum never changes. But the speed of light in any transparent medium is less than the speed of light in a vacuum. Light moves through air slightly more slowly than through a vacuum. The speed of light through water or glass is considerably slower than through a vacuum. The index of refraction is a property of a transparent material defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the material (v). We use n to represent the index of refraction of a material. n n = = speed of light in vacuum speed of light in material c v 434 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

10 Example 10.3 Index of Refraction for Diamond The speed of light in a diamond is m/s. Find the index of refraction of the diamond. Solution: The index of refraction is the ratio of the speed of light in vacuum to the speed of light in the material. Since c = m/s, m/s n = = m/s The index of refraction for diamond is Table 10.1 gives the indexes of refraction for several materials. Note that the index of refraction of air is very nearly that of vacuum. Unless you need to use five significant figures, use for the index of refraction of air. Table 10.1 Index of Refraction Substance n Vacuum Air Ice 1.31 Water 1.34 Quartz 1.46 Crown glass 1.52 Flint glass 1.63 Diamond 2.42 Example 10.4 Speed of Light in Ice Find the speed of light in a cube of ice. Solution: From Table 10.1, the index of refraction of ice is c n = v The speed of light in ice is m/s. c m/s v = = n 1.31 v = m/s 8 8 SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 435

11 Snell s Law of Refraction When light goes from air into glass, it is bent toward the normal, as shown in Figure The light ray in air is called the incident ray, and in glass it is called the refracted ray. The angle of incidence θ 1 is the angle between the incident ray and the normal. The angle of refraction θ 2 is the angle between the refracted ray and the normal. Notice that θ 2 < θ 1 ; the angle of refraction is less than the angle of incidence. Figure Light is refracted toward the normal when it travels from air to glass. What happens when the light ray hits the bottom surface of the glass block? As shown in Figure 10.19, it is bent away from the normal. This time, the incident ray is traveling in the glass and the refracted ray is in air. Notice that the angle of refraction is greater than the angle of incidence. 436 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS Figure Light is refracted away from the normal when it travels from glass to air. The general relationship between angles of incidence and refraction was found by the Dutch scientist Willebrod Snell in Snell s law can be written as a ratio as follows: sin θ sin θ 1 2 = n. n2 1

12 If you cross multiply, n 1 sin θ 1 = n 2 sin θ 2 In Snell s law, n 1 is the index of refraction of the medium for the incident ray and n 2 is the index of refraction of the medium for the refracted ray. Thus, from Snell s law, if light travels from air to glass (n 2 > n 1 ), it is refracted toward the normal (sin θ 2 < sin θ 1 or θ 2 < θ 1 ). If light travels from glass to air (n 2 < n 1 ), it is refracted away from the normal (θ 2 > θ 1 ). Example 10.5 Using Snell s Law Sunlight shines on the smooth surface of a lake with an angle of incidence of 40º. What is the angle of refraction? Solution: From Table 10.1, n 1 = 1.00 and n 2 = Use Snell s law. n1 sin θ1 = n2 sin θ2 (. 100) sin (40 ) = (. 134) sin θ2 sin θ2 θ sin 40 = = = In the result for Example 10.5, is the light ray bent toward or away from the normal? Is this the correct direction? In reality, the index of refraction of a particular material is slightly different for different wavelengths of light. Blue light passing from air into glass, for example, is bent a little more than red light. Sunlight, or white light, contains all wavelengths of visible light. When white light passes through a prism (a piece of glass of triangular cross section) the blue part of the white light is bent the most. The red part is bent the least. The prism separates the white light into its different colors, giving the familiar rainbow effect. 2 SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 437

13 Total Internal Reflection When light passes from a medium of higher index of refraction into air, the light is refracted away from the normal. For example, when a light ray passes from water into air, the angle of refraction is greater than the angle of incidence. When you see a fish beneath the surface of a pond, its actual and apparent locations are not the same. Figure shows that, as the angle of incidence increases, the angle of refraction increases, until at some point the angle of refraction reaches 90. The angle of incidence is called the critical angle at this point. We use θ c to represent the critical angle. When the measure of the angle of incidence exceeds the measure of θ c, the incident ray does not leave the water it is reflected at the surface, not refracted. This is called total internal reflection. When light from the fish undergoes total internal reflection, you can no longer see the fish from the air. What is the measure of θ c for light traveling from water into air? From Snell s law n water sin θ 1 = n air sin θ 2 when θ 1 = θ c,θ 2 = 90. Figure When the angle of refraction is greater than 90, a light ray is reflected, not refracted. This is called total internal reflection. sin θ θ c c n = air = sin )(. 100) = = n 133. water When light travels from water into air, total internal reflection occurs when the angle of incidence exceeds CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

14 Total internal reflection has an important application in optical fibers. These devices transmit optical signals for data, audio, and video for computer networks and the telecommunication industry. The core of an optical fiber is a long, thin strand of transparent glass or plastic. Light signals from lasers are transmitted by the fibers. As the light travels through the fiber s core, it strikes the surface of the fiber with an angle of incidence greater than the critical angle for the glass or plastic. The light stays in the fiber since each interaction is total internal reflection. Figure Optical fibers use total internal reflection to transmit light signals. Lenses A lens is a carefully ground or molded piece of transparent material such as glass or plastic. Lenses are used in many familiar optical systems to change the directions of light rays. These systems include cameras, microscopes, telescopes, binoculars, contact lenses, eyeglasses, and magnifying glasses. Your eye contains a lens. Sharp vision in each eye depends on the ability of your lens to accurately focus light. A lens has two surfaces. Each surface is part of a sphere. We will study two types of lenses convex and concave. A convex lens is thicker at its center than at its edges. This shape causes a convex lens to be a converging lens. When parallel rays of light pass through a convex lens, they are refracted so they meet at a point (or converge) on the other side of the lens. A concave lens is thinner at its center than at its edges. Concave lenses are diverging lenses. Parallel rays spread apart (or diverge) when they pass through a concave lens. (a) Convex (converging) lens (b) Concave (diverging) lens Figure Convex and concave lenses bend light rays. SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 439

15 Focal Point and Focal Length for Thin Lenses When light passes through a lens, it is refracted at each surface. You can use Snell s law to predict the change in the path of each light ray at each surface. Alternatively, you can use the thin-lens approximation. A thin lens is one in which the thickest part of the lens is small compared to the focal length of the lens. With this approximation, you model all refraction of light with a single change in direction occurring on the plane at the center of the lens. All lenses in this book are thin. In addition, we will study symmetric lenses only. In a symmetric lens, the radius of curvature of one face equals the radius of curvature of the other face. Figure uses the thin-lens approximation to illustrate the focal points and focal length of a convex lens. In Figure 10.23a, incident parallel rays on the left are refracted and then converge at a point F on the right. This point is called the focal point. The distance from the center plane of the lens to the focal point is the focal length f. The convex lens also has a focal point on the left. As shown in Figure 10.23b, when light rays leave this point, the lens refracts the diverging rays so that they emerge as parallel rays. Since the lense is symmetric, the focal length on the left equals the focal length on the right. (a) Incident parallel rays are refracted to converge at the focal point on the opposite side. (b) Incident diverging rays from the focal point are refracted as parallel rays on the opposite side. Figure Focal points F and focal length f for a convex lens Figure shows the focal points and focal length for a concave lens. When parallel rays are incident, as in Figure 10.24a, the lens spreads the rays apart. If the directions are traced backward to the left side of the lens, it appears that the rays diverge from a single point. This is the focal point on the left side of the concave lens. The lens also has a focal point on the right side. When incident rays converge at this point, the lens diverges the rays and makes them parallel. Since the lens is symmetric, the focal lengths on the left and right are equal. 440 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

16 (a) Incident parallel rays are refracted to diverge on the opposite side. Each ray appears to come from the focal point. (b) When incident rays appear to converge at the focal point, they are refracted as parallel rays. Figure Focal points F and focal length f for a concave lens Effect of Lens Diameter The greater the diameter of a lens, the greater the amount of light that passes through it. A larger lens will catch more light in the same way that a wider-mouthed jar is likely to catch more rainwater. A lens that s twice the diameter of another lens catches four times as much light. (Can you explain why?) Large astronomical telescopes, such as the 82-inch Otto Struve telescope at McDonald Observatory, are big so they can capture as many photons from distant stars as possible. Figure shows two lenses. They both collect the light that passes through them and focus it to a point. However, more light is being focused by the larger lens. In the same way, the larger the lens used to form an image of an object, the brighter that image is likely to be. That s because it receives and refracts more of the light coming from the object. The size of the lens in our eye doesn t change. But the opening that allows light into the eye the pupil can change. In a darkened room, the pupil is relatively large (about 8 mm in diameter). Most of the eye lens is exposed to light. On a sunny day, the pupil is smaller (about 2 mm in diameter). Less of the eye lens is exposed to light. Figure A lens twice the diameter of another lens refracts four times as much light. SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 441

17 Images Formed by Lenses The most important use for lenses is to form images. As shown in Figure 10.26, you can use ray tracings similar to those used for mirrors to predict the location and size of an image. An object is located on the optical axis to the left of a convex lens. Ray 1 travels from point P on the object parallel to the optical axis. The lens refracts ray 1 so that it passes through the focal point. Ray 2 passes through the focal point on its way to the lens. The lens refracts ray 2 so that it is parallel to the optical axis. The point of intersection of ray 1 and ray 2 is the point P in the image. 442 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS Rays from all other points on the object would be refracted to the corresponding image points on the image. The image created is real it can be seen if a screen is placed at its location since light actually travels to this location. The image is also inverted and smaller than the object. The object distance s o, image distance s i, and focal length f of the lens are related by the lens equation = s s f o Figure Ray tracing of image formed by a convex lens Notice that the lens equation is the same as the mirror equation. Use the following signs (+ or ) for s o, s i, and f in the lens equation: 1. The object distance s o is positive on the object side of the lens. 2. The image distance s i is positive on the image side of the lens (where images are real). It is negative on the object side (where images are virtual). 3. The focal length f is positive for convex lenses and negative for concave lenses. For example, in Figure 10.26, s o, s i, and f are all positive. In Figure 10.24a, s o is positive but s i and f are negative. i

18 Example 10.6 Focal Length of a Convex Lens An object is located 40.0 cm from a convex lens that has a focal length of 22.0 cm. Where should you place a screen to see the image? Solution: Since this is a convex lens and it produces a real image, s o, s i, and f are all positive = so s f i = = si f so 22.0 cm 40.0 cm = = si (22.0) (40.0) 880 cm 880 si = cm = 48.0 cm 18.0 A real image is formed 48.0 cm from the center of the lens. The image formed by a lens can be smaller than the object (as in Figure 10.26) or larger than the object (as in Example 10.6). The ratio of the image size to the object size is called the magnification of the image. Using similar triangles on a ray diagram, you can show that magnification M depends on only the image distance s i and object distance s o : image size Magnification = object size M si = s o A negative magnification means the image is inverted. In Example 10.6, the magnification is si 48 M = s = = 1.2 o 40 The image in Example 10.6 is an enlargement since M > 1. The image is inverted since M is negative. SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 443

19 Example 10.7 Size of a Virtual Image A convex lens in a magnifying glass has a focal length of 8.0 cm. Suppose you hold the lens 6.0 cm from a 0.5-cm ant. (a) Make a ray-trace drawing showing the object and image. (b) How large is the image of the ant? Solution: (a) As before, draw two incident rays from the object one parallel to the optical axis and one through the focal point. Ray 1 is refracted to pass through the focal point on the right. The object is located between the focal point and the lens, so, to find the direction of ray 2, extend an imaginary line to F. Notice that ray 1 and ray 2 diverge as they leave the lens. Extending the rays to the left, it appears as if they originate from a point on the virtual image. (b) Find the image distance s i, and then use the equation for magnification. Find s i from the lens equation = so s f i = = s f s i o 80. cm 60. cm = = 20. si (8.0) (6. 0) 48 cm 48 si = cm = 24 cm 20. The image is virtual since s i is negative. 444 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

20 si ( M = = 24 cm) = 40. so 60. cm image size = M object size Image size = M (object size) = 4005.(. cm) = 20. cm The virtual image of the ant is 2.0 cm long. Summary An object is a source of light rays that diverge from the object. Ray optics is a method of predicting the locations of images by modeling the paths followed by light rays as straight lines. The law of reflection states that, when light reflects from a surface, the angle of reflection is equal to the angle of incidence. A spherical mirror has a focal point on the optical axis midway between the mirror and the center of curvature. (F = r/2) For a concave mirror, incident parallel rays are reflected to converge at the focal point. For a convex mirror, incident parallel rays are reflected to diverge as if they originated at the focal point on the opposite side of the mirror. For mirrors and lenses, the focal length, object distance, and image distance are related by the mirror equation and the lens equation: = s s f o The index of refraction of a material is the ratio of the speed of light in vacuum to the speed of light in the material. Light is refracted at the interface of two materials. Snell s law states the relationship between the angles of incidence and refraction and the indexes of refraction. n 1 sin θ 1 = n 2 sin θ 2 si The magnification provided by a lens or mirror is given by M = s. i. o SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 445

21 Exercises 1. The law of reflection states that the angle of equals the angle of. 2. Light strikes a vertical surface from the left with an angle of incidence of 30º. Sketch the surface, normal to the surface, an incident light ray, and the reflected light ray. Label the measures of the angles of incidence and reflection. 3. In an optics lab, how can you prove that an image is a real image? 4. Suppose you have extremely sensitive photographic film and you can expose it in an optical system for a very long time. Will you be able to record a virtual image on the film? Explain. 5. A concave mirror has a radius of curvature of 12 cm. What is the focal length? 6. A convex mirror has a focal length of 8.2 cm. What is the radius of curvature? 7. A 1.5-cm-high arrow is located 11.0 cm in front of a concave mirror that has a 6.0-cm center of curvature. (a) Make an accurate drawing of the mirror, optical axis, and object. Locate the focal point on the optical axis. (b) Use a ray diagram to find the image of the arrow. From your drawing, what is the image distance s i? Is the image real or virtual? (c) Calculate the image distance from the mirror equation, and compare to the measured distance. (d) The magnification of the image is defined as the ratio of the size of the image to the size of the object: image size Magnification = object size What is the magnification for your ray diagram? 446 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

22 8. A 2.0-cm-high arrow is located 5.0 cm in front of a convex mirror that has a 4.0-cm focal length. (a) Make an accurate drawing of the mirror, optical axis, and object. Locate the focal point on the optical axis. (b) Use a ray diagram to find the image of the arrow. From your drawing, what is the image distance s i? Is the image real or virtual? (c) Calculate the image distance from the mirror equation, and compare to the measured distance. (d) What is the magnification (from your ray diagram)? 9. Use Table 10.1 to find the speed of light in the following: (a) a block of ice (b) a quartz window (c) a crown-glass window 10. What is the index of refraction of a plastic if the speed of light in the plastic is m/s? 11. Sunlight strikes the surface of a lake at an angle of incidence of 55.0º. What is the angle of refraction? 12. The speed of light in a clear liquid is m/s. When light is incident on the surface of the liquid, the angle of refraction is 36.2º. What is the angle of incidence? 13. Light traveling in air strikes a vertical surface of a block of glass from the left with an angle of incidence of 25.0º. The angle of refraction is 15.5º. (a) Sketch the vertical surface, normal, incident ray, and refracted ray. Label the measures of the angles of incidence and refraction. (b) What is the index of refraction of the glass? 14. The sides of an aquarium are made of glass, whose index of refraction is As shown at the right, light traveling in the water inside the aquarium strikes the glass with an angle of incidence of At what angle θ 3 does the light enter the air? SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 447

23 15. What is the critical angle for light traveling from crown glass into water? 16. What is the critical angle for light traveling from water into crown glass? 17. One way of determining a material s index or refraction is to measure the critical angle for light traveling from the material into air. Calculate the index of refraction of plastic that has a critical angle of 43.8º. 18. A convex lens has a focal length of 3.0 cm. An object is placed 7.0 cm to the left of the lens. Make a ray-trace diagram to find the location of the image. 19. Two lenses have exactly the same sizes and shapes, but they have different focal lengths. What is different about the two lenses? 20. A copy machine uses a convex lens with a focal length of 20.0 cm. (a) The machine is set to makes copies (images) of printed pages that are the same size as the originals (objects). What is the relationship between the object distance s o and image distance s i in the machine? (b) Calculate the values of s o and s i. 21. The setting for the copy machine of exercise 20 is adjusted so the original is 32.0 cm from the lens. (a) To make a copy, the machine prints the image on a blank sheet of paper. How far from the lens should the blank paper be located? (b) What is the magnification? Is the copy an enlargement or a reduction of the original? 22. A magnifying glass has a convex lens with a focal length of 16.0 cm. A stamp collector uses the glass to view a stamp placed 8.6 cm from the lens. (a) How far does the image of the stamp appear to be from the lens? (b) Is the image real or virtual? (c) If the stamp has a width of 1.20 cm, how wide is its image? 23. A prism made of crown glass is in the shape of an equilateral triangle. Each side of the triangle measures 5 cm. A laser beam is directed into the midpoint of one side, parallel to the adjacent side. Make an accurate ray-trace drawing of the laser beam through the prism. Find each angle of incidence and angle of refraction. Label the angles on your drawing. 448 CHAPTER 10 LIGHT AND OPTICAL SYSTEMS

24 24. Ray tracing through a concave lens is similar to that for a convex lens. But remember, a concave lens diverges incident rays. An incident ray parallel to the optical axis is refracted in a direction it would have if it originated at the focal point. An incident ray traveling toward the opposite focal point is refracted parallel to the optical axis. The intersection of the lines containing these rays shows the apparent origin of the refracted rays. A nearsighted person uses concave lenses in eyeglasses to see clearly. An object 20 m away forms a virtual image at an image distance of 15 m. What is the focal length of the lens? 25. A 50-mm camera lens means the focal length of the lens used by the camera is 50 mm. The lens refracts light from an object onto film in the back of the camera. The lens moves forward or backward along the optical axis to adjust the image distance, so the image is focused on the film. (a) A 50-mm camera lens focuses on an object 3 meters in front of the lens. What is the image distance? The lens is refocused on an object 30 meters away. What is the new image distance? How far (in millimeters) is the lens moved to refocus on the second object? (b) The camera lens is replaced with a 200-mm lens. This lens focuses on an object 3 meters away. Then the lens is refocused on an object 30 meters away. How far (in millimeters) is this lens moved to refocus on the second object? SECTION 10.1 RAY OPTICS: REFLECTION AND REFRACTION 449